A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A lithographic apparatus typically includes an illumination system, which is arranged to condition radiation generated by a radiation source before it is incident upon the patterning device. The illumination system may, for example, modify one or more properties of the radiation such as polarization and/or illumination mode. The illumination system may include a uniformity correction system which is arranged to correct or reduce non-uniformities present in the radiation. In some instances the uniformity correction system may compromise or modify in an unwanted manner one or more properties of the radiation beam.